There has been known a projection image display device (projector) for separating light emitted from a light source into three colors of R (red), G (green), and B (blue), optically modulating the respective color lights by a liquid crystal panel according to an image signal, synthesizing again the optically modulated color lights, and enlarging and projecting an image of the synthesized color. The projection image display device includes an illumination optical system (optical image forming unit) for irradiating the liquid crystal panel with light emitted from the light source to form an optical image, and an enlargement projection system for enlarging and projecting the optical image emitted from the illumination optical system. Further, there has also been proposed a projection image display device (compound projector) including two such illumination optical systems, in which optical images emitted from each of the illumination optical systems are synthesized and the synthesized optical image is projected from one enlargement projection system. For example, Patent Document 1 discloses a configuration having two illumination optical systems, and an adjustment mechanism for adjusting the positional relationship between the two illumination optical systems. Further, Patent Document 2 discloses a configuration for synthesizing each of the color lights by converting R, B lights to S polarized light and by converting G light to P polarized light. The proposed configuration is also designed to synthesize the optical images from the two illumination optical systems by adjusting one optical image to S polarized light, and by adjusting the other optical image to P polarized light. Further, Patent Document 3 discloses a configuration having two illumination optical systems, in which image lights of different polarizations from two liquid crystal panels are synthesized by a half mirror, and a three-dimensional image is projected on a screen.